We check tx avail through vhost_enable_notify() in the past which iswrong since it only checks whether or not guest has filled moreavailable buffer since last avail idx synchronization which was justdone by vhost_vq_avail_empty() before. What we really want is checkingpending buffers in the avail ring. Fix this by callingvhost_vq_avail_empty() instead.

This issue could be noticed by doing netperf TCP_RR benchmark asclient from guest (but not host). With this fix, TCP_RR from guest tolocalhost restores from 1375.91 trans per sec to 55235.28 trans persec on my laptop (Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz).